Investment Securities (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Schedule of Available-for-sale Securities [Line Items]
Total securities available-for-sale	$ 3,942,708	$ 4,017,909
Available-for-sale Securities, Gross Unrealized Gain	64,806	31,812
Available-for-sale Securities, Gross Unrealized Loss	(12,806)	(40,290)
Securities available‑for‑sale	3,994,708	4,009,431
Government agency and government-sponsored enterprise pass through securities
Schedule of Available-for-sale Securities [Line Items]
Total securities available-for-sale	340,184	281,486
Available-for-sale Securities, Gross Unrealized Gain	3,990	1,902
Available-for-sale Securities, Gross Unrealized Loss	(998)	(2,300)
Securities available‑for‑sale	343,176	281,088
Government agency and government-sponsored enterprise collateralized mortgage obligations
Schedule of Available-for-sale Securities [Line Items]
Total securities available-for-sale	712,090	634,774
Available-for-sale Securities, Gross Unrealized Gain	8,625	3,448
Available-for-sale Securities, Gross Unrealized Loss	(2,939)	(5,372)
Securities available‑for‑sale	717,776	632,850
Private label collateralized mortgage obligations
Schedule of Available-for-sale Securities [Line Items]
Total securities available-for-sale	121,981	101,313
Available-for-sale Securities, Gross Unrealized Gain	2,205	1,985
Available-for-sale Securities, Gross Unrealized Loss	(939)	(2,093)
Securities available‑for‑sale	123,247	101,205
Municipal securities
Schedule of Available-for-sale Securities [Line Items]
Total securities available-for-sale	1,145,963	1,298,514
Available-for-sale Securities, Gross Unrealized Gain	40,748	21,000
Available-for-sale Securities, Gross Unrealized Loss	(1,710)	(7,320)
Securities available‑for‑sale	1,185,001	1,312,194
Government agency and government-sponsored enterprise commercial mortgage-backed securities
Schedule of Available-for-sale Securities [Line Items]
Total securities available-for-sale	1,079,816	1,133,846
Available-for-sale Securities, Gross Unrealized Gain	4,460	383
Available-for-sale Securities, Gross Unrealized Loss	(5,365)	(21,525)
Securities available‑for‑sale	1,078,911	1,112,704
US Treasury Securities
Schedule of Available-for-sale Securities [Line Items]
Total securities available-for-sale	287,721	401,056
Available-for-sale Securities, Gross Unrealized Gain	2,996	2,437
Available-for-sale Securities, Gross Unrealized Loss	0	(88)
Securities available‑for‑sale	290,717	403,405
Asset-backed Securities
Schedule of Available-for-sale Securities [Line Items]
Total securities available-for-sale	185,592	81,762
Available-for-sale Securities, Gross Unrealized Gain	80	104
Available-for-sale Securities, Gross Unrealized Loss	(516)	(481)
Securities available‑for‑sale	185,156	81,385
SBA asset-backed securities
Schedule of Available-for-sale Securities [Line Items]
Total securities available-for-sale	52,361	68,158
Available-for-sale Securities, Gross Unrealized Gain	2	0
Available-for-sale Securities, Gross Unrealized Loss	(339)	(1,111)
Securities available‑for‑sale	52,024	67,047
Corporate debt securities
Schedule of Available-for-sale Securities [Line Items]
Total securities available-for-sale	17,000	17,000
Available-for-sale Securities, Gross Unrealized Gain	1,700	553
Available-for-sale Securities, Gross Unrealized Loss	0	0
Securities available‑for‑sale	$ 18,700	$ 17,553